Breast cancer is the most common malignant tumor in women in the United States and is second only to lung cancer as a cause of cancer death in women. Despite campaigns to educate women on regular breast self-examination and the widespread use of screening mammography, many breast cancers are not discovered until after they have undergone distant metastasis and the prognosis is poor. Mammography not only has a high false negative rate, which allows many early tumors to go undetected, but also has a high false positive rate resulting in many unnecessary biopsies with their attendant costs and morbidities.
One significant advancement in locating malignant tumors in the body has been the development of PET (Positron Emission Tomography) scanners. PET is a nuclear medicine technology that uses radioisotopes to allow the noninvasive diagnostic imaging of metabolic processes in various organ systems of the human body. Images are obtained from positron-emitting radioactive tracer substances (radiopharmaceuticals) that are usually administered intravenously to the patient. Where as computed tomography (CT) and magnetic resonance imaging (MRI) provide information about anatomic structure, PET can image and quantify biochemical and/or physiological function. This information is potentially valuable because functional changes caused by disease are frequently detectable before any structural abnormalities become evident.
PET scanners currently in clinical use are cylindrical in shape and the patient is passed through the bore of the cylinder during the imaging process. While cylinders may have advantages in terms of simplicity of construction and compact size, they have serious drawbacks as imaging devices for Positron Emission Tomography. Many of the corrections that complicate image reconstruction in PET have their roots in the cylindrical shape of the detector. Most efforts to improve contrast and resolution of cylindrical PET scanners have focused on improving crystal sensitivity and detection electronics as opposed to changing detector geometry.
Breast cancer screening with PET could have the potential to detect malignant tumors much earlier in their natural history than current screening imaging techniques, improving the chances for curative therapy. To justify the increased cost and time required to do screening PET studies for breast cancer, the PET detection system needs to be optimized so the sensitivity and specificity are so clearly superior to conventional imaging techniques that it will be accepted both on clinical and economic grounds by patients and third party payers.
To date, clinical use of PET in breast cancer consists of whole body studies using conventional cylindrical detectors for tumor staging. Even with less than ideal detector geometry, PET has shown greater sensitivity and specificity than any other imaging modality in detecting metastatic disease.
FIG. 1 illustrates a cylindrical PET detector 10 typically employed for whole-body scanning. Cylindrical detectors imaging a mono-energetic point source will record different energy levels for photons since only the slice at the location of the source will detect photons normal to its surface. All other photons 12 will strike the detector surface at an angle θ. In addition, in a cylindrical configuration, there are no counts from the accessible front side of the breast. This reduces sensitivity, contrast and resolution and may small lesions near the areola and nipple.
Dedicated PET mammography prototype devices have been built and are currently being used in research settings. However, these devices use planar scintillation detectors to image the compressed breast during conventional mammography and are therefore subject to the limitations of planar imaging, as they lack the sensitivity to reliably detect the smallest and hence most curable malignancies.
Accordingly, an object of the present invention is to provide an improved non-planar and non-cylindrical PET detection system that provides for higher resolution and contrast in tumor detection so that a malignant tumor can be located at earlier stages of growth, and smaller tumor sizes can be identified.
Another object of the present invention is to provide a mammography PET detection system that improves the detection of breast cancer growths and the capacity to distinguish between malignant and benign breast tumors. At least some of these objectives will be met in the inventions described hereinafter.